In wireless communications, the physical channel between the transmitter and the receiver is formed by a radio link. In most cases, many different propagation paths exist between the transmitter and the receiver. This is due to reflections in the environment, e.g. against buildings, and gives rise to a multipath channel with several resolvable components.
The performance of a Code Division Multiple Access (CDMA) receiver is improved if the signal energy carried by many multipath components is utilized. This is traditionally achieved by using a RAKE receiver. A detailed description of RAKE receivers may be found e.g. in the book “WCDMA for UMTS” by Holma, & Toskala, A. (Wiley 2000).
A RAKE receiver includes various ‘fingers’, or despreaders, each finger having an assigned path delay for receiving a particular image of a multipath signal and a correlator for despreading the received image. In combination, the fingers despread multiple signal images of a received multipath signal, thus mitigating the effect of the multipath channel fading phenomenon.
In other words, in a RAKE receiver, each multipath component is assigned a finger, or despreader, whose reference copy of the spreading code is delayed equally to the path delay of the corresponding multipath component.
The outputs of the RAKE fingers are coherently combined to produce a symbol estimate. Thus, the RAKE receiver requires knowledge of the multipath delays and the values of the channel impulse response for all paths. The best performance is achieved if the signal energy from all paths is utilized.
The combining of the outputs performed by the RAKE receiver improves the signal-to-noise ratio (SNR) since it allows the desired signal components to be summed coherently, while the interference and noise components are summed non-coherently. When the noise components are uncorrelated at each RAKE finger, they partially cancel each other out, while the signal components are rotated so as to sum constructively.
The combining is made by a Maximal-Ratio Combining (MRC) procedure, wherein the different signal components are weighted according to their respective SNR.
An implicit assumption is made in the MRC weight computation process that the noise and interference components on each finger are uncorrelated. The MRC combining produces the best SNR if the RAKE fingers are placed on the strongest paths, i.e. on the delays corresponding to the paths that have instantaneously largest magnitudes at a given time.
On the other hand, it is disadvantageous to place a RAKE finger on a sidelobe of a path that is already covered, since the impairment components would be correlated and no diversity advantage would be gained, as the sidelobe fades together with the main lobe.
In order to improve the RAKE receiver performance, a variety of advanced receiver types have been developed. One such advanced receiver type is the dual-antenna receiver, where the signal is received via two separate antennas, RF (Radio Frequency) and front end processing branches. If the two branches are sufficiently separated (electrically and spatially), the fading processes and the noise and interference signal components on the two branches, as seen by the RAKE receiver, will be substantially uncorrelated.
The dual-antenna RAKE receiver (RAKE2) will then exhibit improved performance, due to the array gain (more signal energy is received) and the diversity gain (reduced probability of deep fades). As a result, the Block Error Rate (BLER) performance of the receiver will be improved.
For the RAKE2 receiver, under the assumption of independent fading and impairment signals, the finger placement is the same as in a single-antenna receiver. The RAKE2 fingers are placed on the strongest paths, without regard to which antenna branch they belong to. For a well-separated multi-branch architecture, this would be the best way of utilizing the available diversity and array gain.
However, for a practical mobile terminal design, the assumptions of independent fading and uncorrelated impairment signals are not always justified. Unfortunately, a number of “cross-talk” mechanisms exist that may introduce significant correlation between the signals of the individual receiver branches.
Firstly, cross-talk may occur due to spatial limitations, i.e. that the two receiver antennas may not be sufficiently separated due to terminal size constraints.
Secondly, electrical coupling may occur when the circuits and conducting surfaces in the mechanical design of the terminal introduce coupling between the signal paths of the receiver branches.
Finally, user body interaction may play a part, since depending on the user hold of the mobile terminal, considerable correlation between the antennas may be introduced that cannot be accounted for in the original design.
All these mechanisms give rise to increased correlation between the effective fading trajectories of the two channels and between the received noise plus interference signal components, as seen by the RAKE2 receiver.
More specifically this means that two RAKE fingers placed at the same delay on the different antennas would no longer experience independent fading and noise plus interference. As a result, the MRC combining would not produce the expected increase in SNR, and may, in fact result in a degradation of the SNR after combining. There are two main reasons for this potential degradation.
Firstly, if the fading is highly correlated, placing more than one finger per fading process (path) makes fewer fingers available to cover other, independent fading processes. This in turn means that the diversity gain is not utilized.
Secondly, if the impairment components on the two fingers are correlated, they accumulate coherently, together with the signal component, which in turn means that the array gain is compromised.
Hence there is a need for a receiver with improved handling of antenna correlation related issues.